PD does two big things:

  1. Alerting: Phones you when your servers are down.
  2. Incident Management: Help coordinate a response across multiple people.

For the first, there's also:

  - OpsGenie (owned by Atlassian)
  - Squadcast
  - VictorOps (now Splunk On Call)
  - xMatters
  - PagerTree

For the second, there's a bunch of new contenders:

  - Datadog now has an IM product
  - Blameless
  - Rootly
  - Incident.io
  - FireHydrant